This application claims the priority of Swiss patent application 2535/98, filed Dec. 22, 1998, the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a method and a sensor for measuring the mass flow of a gas or a liquid according to the preamble of the independent claims.
Devices of this type are described in xe2x80x9cScaling of Thermal CMOS Gas Flow Microsensors: Experiment and Simulationxe2x80x9d by F. Mayer et al., in Proc. IEEE Micro Electro Mechanical Systems (IEEE, 1996), pp. 116ff or in xe2x80x9cSingle-Chip CMOS Anemomenterxe2x80x9d by F. Mayer et al., Proc. IEEE International Electron Devices Meeting (IEDM, 1997), pp. 895ff. They are used for determining the mass flow of gases or fluids. They contain a heating element, which is arranged between two temperature sensors. The temperature difference between the two temperature sensors is an indicator for the mass flow.
Such sensors have a comparatively large power consumption. It is therefore endeavored to keep the current through the heating element as low as possible, at the expense, however, of accuracy.
U.S. Pat. No. 4,651,564 describes a sensor where the heating element is operated in pulsed manner, wherein the pulses are as long as the pauses between the pulses. This pulsed operation serves to increase measuring accuracy. A reduction of power consumption is, however, not an object of this document.
EP-A-698 786 describes a device comprising a heating element operated by differing heating currents. The heating element is used as temperature sensor at the same time. Measurements at differing heating currents are required for determining the flow to be measured.
It is therefore an aim to provide a sensor and a method, respectively, of the type mentioned initially that operate with a low power consumption.
This aim is reached by the object of the independent claims.
According to the invention the heating element is therefore operated in cycles, wherein in a first, shorter cycle phase more heating occurs than in a second, longer cycle phase. Thus, it can e.g. remain switched off in the second cycle phase, or be operated with lower power.
By means of this pulsed operation the power consumption can be reduced substantially. Still it remains possible, by processing the two sensor signals, to determine the mass flow. Preferably, the pulse length or switch-on time of the heating element is chosen for this purpose such that a thermal equilibrium is reached and the mass flow can be determined in substantially conventional manner.
In a preferred embodiment the sensor is provided with a monitoring circuit, which periodically checks the temperature signals. As soon as these temperature signals fulfil a given threshold condition, a measuring circuit is put into operation, which accurately determines the mass flow. In this way the power requirements can be reduced further.
Preferably, differing modes of operation are provided. In a first mode with reduced power consumption, only very short heating pulses are generated, while the pulses in the second heating mode are longer. This allows to use, if necessary, a mode of operation with reduced power consumption. Even though only measurements with reduced accuracy are possible in this mode of operation, this is sufficient for many applications. The mode of operation with increased power consumption is only used if measurements of higher accuracy are necessary.
Preferably, the heating pulses are at least five times, preferably ten times, shorter than the distance between consecutive heating pulses.